Rotational state detection of electrically trapped polyatomic molecules

Abstract

Detecting the internal state of polar molecules is a substantial challenge when standard techniques such as resonance-enhanced multiphoton ionization or laser-induced fluorescense do not work. As this is the case for most polyatomic molecule species, in this paper we investigate an alternative based on state-selective removal of molecules from an electrically trapped ensemble. Specifically, we deplete molecules by driving rotational and/or vibrational transitions to untrapped states. Fully resolving the rotational state with this method can be a considerable challenge, as the frequency differences between various transitions are easily substantially less than the Stark broadening in an electric trap. However, by using a unique trap design that provides homogeneous fields in a large fraction of the trap volume, we successfully discriminate all rotational quantum numbers, including the rotational M-substate.substate alone is only possible for M = 1, for > M 1 it is at least theoretically possible to deplete two sets of rotational states, which differ by the single M-substate of interest.As an example, we discuss the depletion of the single M-substate | + = + 〉 J K M J 1; ;1 , using MWD and IRD (see figures 1 and 2). In both cases, the MW coupling,, is the only one that addresses the | + = + 〉 J K M J 1; ;1 state. The population of this state is detected by running the whole experimental sequence twice: the depletion (MWD or IRD) is applied once with driving the1; ; 1 transition, and once without. The difference of both measurements yields the population in the | + = + 〉 J K M J 1; ;1 state. An extension to other single M-substates is possible, but at the same time more challenging due to experimental limitations. Using MWD, any single M-substate can be detected by coupling all M-substates up to the desired single M-substate and measuring twice, as explained above. To be able to implement this scheme, a sufficient spectral resolution is needed to resolve all individual transitions. In the case of IRD, one has to keep in mind that spontaneous decay can lead to population transfer to the state of interest. J = 3 and J = 5 states used for our depletion schemes, as shown in figure 4.IR transition frequencies of the v 1 vibrational mode are provided in [47] and were verified via saturated absorption spectroscopy in a room-temperature cell. The transition frequencies were determined with sub-MHz resolution using a frequency comb. We found that, luckily, the Q-branch IR transition from the = | | = J K 3, 3 state is well isolated, with the closest relevant transition from a different state being several GHz away (see figure 6). Our spectral resolution is clearly better than this, and thus the driving of this transition is K and J selective. The IR transition from the = | | = J K 5, 3 state is less isolated. Here, within several GHz we find also the IR transitions from the = | |= J K 7, 8, 2 states and = | |= J K 8, 9, 10, 1 states. However, inside our trap, these states are in sum populated by at most a few percent, and can...

Thermometry of Guided Molecular Beams from a Cryogenic Buffer‐Gas Cell

“…Several techniques for state detection of cold molecules have been used so far [34,35,36,37,38,39,40]. Among them, depletion methods, as adopted in previous experiments carried out by our group [35,40], are particularly advantageous since they are applicable to a large range of molecules and avoid the difficulties of the light-induced fluorescence (LIF) and resonance-enhanced multi-photon ionization (REMPI) detection techniques. Along this line, we have extended the depletion technique by implementing rotational-state detection adapted to the cold guided molecular beams from a cryogenic buffer-gas source, using resonance RF depletion spectroscopy in a parallel-plate capacitor.…”

Rotational Cooling of Trapped Polyatomic Molecules

“…In fact, we even expect ∼90 % of the molecules to populate this particular rotational state, as our detection method underestimates the real occupation by roughly 20 % [27]. The achieved purity is mainly limited by the above discussed blackbody radiation, which removes molecules from the target state during application of SSP and cleaning with a rate of 0.08 Hz.…”

“…The narrow electric-field distribution of the trap [27] transitions, with minimal Stark broadening. In addition, the long trap lifetime of up to 30 s [6] is crucial for the implementation of an optical pumping scheme based on spontaneous decays of vibrational excitations with typical decay times of more than 10 ms.…”

“…These loss channels are the presence of a Fermi resonance in the vibrationally excited state [6] and an excitation of a distinct vibrational mode by blackbody radiation (estimated excitation rate 0.08 Hz) [26]. The values for uncooled ensembles overestimate the population relative to the other sequences due to an imperfect state detection [27].…”

Measurements of trap dynamics of cold OH molecules using resonance-enhanced multiphoton ionization

“…The long lifetimes increase the probability that interactions can take place before the molecules are ejected from the trap by collisions with background gas or by photon scattering [4]. There have been many recent experiments that have explored various electrostatic [7][8][9][10] and magnetic [11][12][13] trapping geometries, and others that demonstrated additional in-trap cooling [14][15][16][17][18][19]. Importantly, several experiments have taken advantage of trapped molecules to study the properties of the molecules themselves, including vibrational relaxation of OH [20] and NH [21].…”